As disclosed in JP2007-205968A, a hydrogen flame ionization detector (FID) of this kind is intended to detect ion current caused at a time of introducing a sample gas such as exhaust gas into hydrogen flame by a collector electrode to thereby measure a concentration of hydrocarbon which is a measurement target substance contained in the sample gas based on the ion current detected by the collector electrode. This hydrogen flame ionization detector is used while it is provided on an exhaust gas flow path in which the exhaust gas discharged from such as a vehicle.
Since hydrocarbon (for example, THC) contained in the exhaust gas is adhered to and deposited on an inner surface of an exhaust gas flow path incorporating the hydrogen flame ionization detector, an error component is to be contained in a concentration of hydrocarbon obtained by the hydrogen flame ionization detector. Therefore, conventionally, purge gas is supplied to the exhaust gas flow path and the hydrogen flame ionization detector by a predetermined regular purge process to thereby remove the adhered and deposited hydrocarbon.
However, in the case of performing a regular purge process as mentioned above, an amount of the hydrocarbon adhered to the exhaust gas flow path and the hydrogen flame ionization detector exceeds a permissible range in some cases even before performing a purge process. Then, the measurement of a concentration of a measurement target substance is to be performed remaining in a state of being incapable of addressing by such as a correction until the regular purge process is started.
Meanwhile, there may be a case where an amount of hydrocarbon adhered to the exhaust gas flow path and the hydrogen flame ionization detector is within a permissible range even without performing the regular purge process. In that case, a purge process of the exhaust gas flow path and the hydrogen flame ionization detector is to be performed as a lapse of a predetermined maintenance period, and there arises a problem such that a measurement should be necessarily stopped due to an unnecessary purge process.